The present invention relates to a method and apparatus for non-destructive materials testing, in particular for the detection of red rot and other tree diseases in the trunks of living trees.
The increasing contamination of the air and the growing mechanization of the forest industry have led to an increase in tree disease and to an intensified attack by pests. In the Federal Republic of Germany alone losses of about DM 150 million a year are estimated. These losses are even considerably higher in more heavily forested regions; for example, in Scandinavia they are estimated at about DM 200 to 250 million annualy. In order to keep the ecological and economic losses in bounds it is necessary to recognize and combat them as early as possible.
One particular tree disease which causes much damage is red rot which occurs in needle trees and is caused by the fungus Fomes Annosus. This fungus attacks mainly needle trees and rarely leaf trees. Of the coniferous trees, spruce and pine are primarily endangered. The larch, Douglas fir and other kinds are only secondarily attacked. The fungus finds favorable growth conditions predominantly in standing trees and causes a root rot which, in the pine, leads to the tree dying in its youth while bringing about, in the spruce, a gradually increasing destruction of the wood inside the trunk which greatly reduces the value of its lumber, but does not threaten the life of the tree directly. For this reason, the rot is not easily recognizable from the exterior, although the interior wood is being destroyed or is already destroyed and the destruction is continuing.
Because of these damaging effects the fungus fomes annosus is one of the most significant fungi for the forestry industry since the damage brought about by it can be very considerable, be it that in young plants do not develop at all or that in older standing trees with diseased roots are easily uprooted by the wind. In addition, spruce with rotten trunks often suffer broken trunks. This alone may cause considerable lumber losses. But the most important damage is done by the fungus directly in the wood of the trunk. Since red rot moves upwardly from the bottom of the trunk, it is precisely the economically most valuable parts of the wood which are damaged first so as to be no longer usable as building or sawed lumber, although it can only at best be processed for cellulose production or used as fire wood.
If the disease is recognized in time, damage caused by the rot rising in the trunk can be reduced or kept in bounds by early cutting. This requires proof of the disease on the standing tree, if possible without injuring the tree. However, to obtain this proof in a whole stand or in an individual case is not easy. To date, the ultimate extent and spread of the disease can be determined exactly in the individual case only be felling the tree, and in a stand only by examining a sufficient number of felled sample trees.
Another detection method which avoids the above mentioned, relatively expensive sample fellings is the drill chip analysis, which reveals information on the degree of rotting by way of the coloration of a drill chip taken. The technique is described by S. Lange in the journal "Forstw. Centralbl." 78 (1959), pages 174/180. Generally, this requires several drillings for a reliable diagnosis. The injuries inflicted by this process can then again become the starting point of wound rot or trunk rot.
In another method the resistance of the wood to electric current pulses at different trunk depths is measured. This method is described in the journal "Canadian Journal of Forest Research" 2 (1972) pages 54/56. Holes are also drilled into the trunk and then a probe with two electrodes is inserted into the drill holes. Current is supplied via two probe tips and the resistance offered by the wood to current pulses is measured and recorded as a function of the location of the probe tips. The resistance depends on the ion concentration in the wood, and since rotten wood contains cations to a greater extent than anions, the electrical resistance drops across the rotted areas. The intensity of the red rot and the location of the spot where the resistance drops can only be approximated by this method. This method, too, has the disadvantage of damaging the trees and thus making them susceptible to diseases.
Also known are sonic test methods by which reflected sonic pulses are analyzed by appropriate test setups. They are described in "Mitt.Dtsch.Ges.f. Holzforsch." 38 (1955), pages 8 to 11. Some of these methods measure the reflected residual energy of supersonic waves, others the attenuation as a function of frequency. Subsequently, certain conclusions as to the state of rot are drawn from the measured values. These sonic test methods are able to reveal relatively little information only because the differences in the sound intensity between healthy and diseased trunks are too small. By the same token, when measuring the attenuation as a function of frequency, very small differences only can be detected, with test results widely scattered. In addition, problems arise from coupling the sound transmitter to the trunk. The coupling must be very good and requires an absolutely plane surface. Therefore, for an unobjectionable measurement the standing trunk must be injured so that the operating mode of this method is not entirely non-destructive.
In contrast to these methods, which always require a more or less severe injury of the tree, the non-destructive test methods are of greater interest because they do not damage the wood part of the tree.
One such non-destructive test method is x-raying, as described in the journal "Forstw. Centralbl." 78 (1959), pages 174 to 180. But this method is too expensive and unsuited for forestry practice, the apparatus required being too huge and unwieldy in particular.
Finally, a method has been proposed, as described in "For. Sci." 5 (1959), pages 37 to 47, and in "Wood Sci. and Techn." 2 (1968) pages 128 to 137, which operates with the radioactive isotope thulium as a radiation source and in which the blackening of a film disposed on the tree trunk side opposite the radiation source, by the rays emanating from the radioactive radiation source and penetrating the tree trunk is measured. Depending on the thickness of the tree, this requires an exposure time of 1.5 minutes to 15 hours. After development of the exposed film the blackening as a function of location is measured, and on the basis of this blackening, a curve of rotted spots can be identified by comparison with normal curves of healthy trees.
X-ray methods and methods using radioactive isotopes, in principle, measure the weakening of rays of a certain energy range when passing through the wood. The parameter describing this weakening quantitatively is the weakening coefficient .mu.. On the one hand, it is a function of the energy of the radiation used, and on the other, of the weakening substance S. Therefore, at a fixed radiation energy E, the weakening coefficient .mu. is a suitable numerical measure for the description of certain characteristics of the penetrated substance S. Extensive measurements and investigations of the absorption of gamma rays in wood have shown that the absorption coefficient .mu. is a suitable means of obtaining information as to whether the wood is healthy or changed in its chemical composition by the red rot, and thus also in its physical-technical parameters.
According to the hitherto known methods, the degree of weakening, which equals the ratio of the weakened radiation I to the unweakened radiation I.sub.o, and the thickness of the irradiated layer d, which can be measured externally relatively simply, are the factors determining the total absorption, which equals the integral along the direction of radiation over the numerically non-constant values of the absorption coefficient in the interior of the trunk and by means of which it can be determined numerically whether homogeneous, sound wood or partly red rotted wood is involved.
This method, which is based on the determination of the means absorption coefficient .mu..sub.ges' makes it possible to estimate cavities in the trunk, and in the presence of certain conditions the size of the cavity can be determined in many cases from such extrenal measurements. However, the exact location of a cavity cannot be fixed definitely with such an absorption measurement. Results utilizable in practice can be obtained only under the assumption that a defect found is located in the center of the trunk, but in reality the location of such a defect is uncertain. Consequently, this method is too inaccurate and too unreliable.
The invention now makes available a method and apparatus for its execution which makes possible an accurate and nondestructive as well as reliable materials test, in particular for the detection of red rot and other tree diseases in the trunks of living trees.